Background
Over 1.9 billion adults worldwide are overweight and
>600 million are obese, corresponding to 39% and 13 of world’s adult population, respectively [
1]. Multiple genetic and dietary factors are associated with the development of type 2 diabetes (T2D), although less is know about the role of certain environmental factors such as viral infection. Globally, 130–170 million people are infected with hepatitis C virus (HCV) [
2]. While hepatocytes represent the major site of viral replication, liver disease is not the sole outcome of HCV replication, and extrahepatic complications of HCV infection are common and complicate its management (reviewed in [
3]). HCV infection and interferon-based therapies frequently induce endocrine-metabolic complications, including diabetes [
4,
5]. Indeed, an association between chronic HCV infection and T2D and insulin resistance has been demonstrated consistently (reviewed in [
6]), as well as an increased risk of pancreatic cancer [
7,
8].
Multiple studies have demonstrated HCV replication in several extrahepatic tissues and cell types, suggesting that at least some of the extrahepatic manifestations may be caused directly by the virus (reviewed in [
9]). HCV RNA has been detected in the pancreata of patients with chronic HCV suggesting that viral infection occurs in vivo [
10‐
12]. For instance, Laskus et al. detected negative-sense HCV RNA – a replication intermediate – in 5 of 8 post-mortem pancreatic tissues [
10]. Similarly, Yan et al. detected negative-sense HCV RNA and/or viral antigens in pancreata from multiple individuals [
11]. Virus-like particles have also been observed in pancreatic beta cells from individuals with HCV infection [
13]. Preliminary data suggest that HCV sequences in the pancreas are distinct from those circulating in the periphery, offering further evidence of viral adaptation for efficient replication within the pancreas [
12]. Nonetheless, the particular cell types supporting viral replication in the pancreas have not been evaluated extensively, and in vitro models to examine the impact of viral infection on pancreatic cell function are limited.
This pilot study provides preliminary evidence of viral infection of pancreatic islet cells in vitro. This system will be valuable for future studies that further characterize the viral and host factors that facilitate HCV infection of the pancreas and for exploring the mechanisms by which HCV infection may promote the development of diabetes and insulin resistance.
Methods
Cell culture
Human hepatocyte (Huh7.5) and human embryonic kidney (293 T) cell lines were provided by Apath LLC (St. Louis, MO) and maintained in Dulbecco’s Modified Eagle’s Medium (DMEM) high glucose supplemented with 10% fetal bovine serum (FBS), penicillin (100 U/mL), streptomycin (100 mg/mL), and non-essential amino acids.
All human samples (islets) were received from the Integrated Islet Distribution Program (IIDP) [
14] and were de-identified / anonymous to the study investigators. The study was reviewed and approved by the Icahn School of Medicine Institutional Review Board as exempt (GCO#: 09–1593). Donors had no evidence of type 1 or type 2 diabetes. Islets were maintained in RPMI 1640 supplemented with glucose 5.5 mM and 10% FBS. Islets and Huh7.5 cells were cultured at 37 °C in 5% CO
2, and medium was replaced every 2–4 days.
mRNA expression of HCV entry factors
Total RNA from 3 different human islets donors was isolated using TRIzol reagent (Thermo Scientific) in combination with the RNeasy Mini kit (Qiagen) followed by DNase treatment. Five hundred nanograms of total RNA were retrotranscribed using the Superscript III kit (Thermo Scientific), and the cDNAs obtained were utilized as templates for quantitative real-time RT-PCR analysis of CD81 (72 base pair [bp]), occludin (63 bp), claudin-1 (101 bp), and SR-B1 (50 bp), as well as the housekeeping gene GAPDH (94 bp). cDNA was run on an AbiPRISM 7300 fast real-time cycler using the power SYBR Green real-time PCR master mix kit and quantified by built-in SYBR Green Analysis. All samples were evaluated in triplicate with the average relative amount of specific mRNA being normalized to glyceraldehyde 3-phosphate dehydrogenase (GAPDH) expression. The mRNA levels in Huh7.5 cells and human islets are shown relative to those in 293 T cells.
CD81 staining of islet cells
Islet cells were cultured overnight and dispersed using standard enzymatic or non-enzymatic protocols and stained for viability. Surface CD81 or IgG1 isotype and intracellular insulin expression were evaluated by flow cytometry. Plots were gated on live cells with high FSC/SSC properties typical of endocrine cells.
Production of infectious HCV particles and infection of islet cells
The Huh7.5
JFH1 cell line – which releases infectious HCV genotype 2a virions into the cell culture supernatant – was provided by Dr. Guangxiang Luo [
15] and maintained in DMEM with 10% FBS and 5 μg/mL of blasticidin. Infectious virions (hereafter referred to as JFH1) were harvested from the supernatants of Huh7.5
JFH1 cells, filtered, spun at high speed to pellet cellular debris, and stored at −80 °C prior to use. For all experiments, 1 × 10
5 islet cells or Huh7.5 cells were infected for 4 h with 0.5 TCID
50 of virus in a 24-well plate. Virus was then removed, and cells were washed with PBS multiple times to remove unbound virus. Given the limited availability of patient-derived islet cells, each experiment was performed using islet preparations from at least 2 donors. All results shown reflect representative experiments with error bars showing replicate infections within the same islet cell preparation.
Qualitative strand-specific reverse transcription (RT)-PCR
RNA from cell lysates was extracted using TRIzol (Invitrogen; Carlsbad, CA), washed, and resuspended in 50 uL of DEPC-treated dH
2O. RNA from 140 uL of culture supernatant was extracted using the QIAamp Viral RNA Kit (Qiagen; Valencia, CA), and eluted in 60 uL of DEPC-treated dH
2O. HCV RNA was detected utilizing two qualitative strand-specific RT-PCR assays as described previously [
16,
17]. PCR primers included the HCV-II sense primer (5’-CAC TCC CCT GTG AGG AAC T-3′, nucleotides [nt] 38–56 of the 5′UTR) and the HCV-I antisense primer (5′-TGG ATG CAC GGT CTA CGA GAC CTC-3′, nt 342–320) or the antisense primer KY78 (5’-CTC GCA AGC ACC CTA TCA GGC AGT-3′, nt 311–288) and sense primer KY80 (5’-GCA GAA AGC GTC TAG CCA TGG CGT-3′, nt 68–91). 30 cycles of PCR (94 °C for 30 s, 58 °C for 1 min, and 72 °C for 2 min) were performed, and PCR products (295 base pairs in length for HCV-I/-II and 244 base pairs for KY78/80) were visualized by gel electrophoresis.
ELISA for HCV proteins
HCV core protein was quantified in cell culture supernatants by the QuikTiter HCV Core Antigen ELISA Kit (Cell Biolabs, Inc.; San Diego, CA), while HCV NS3 was quantified in cell lysates using the Quantitative HCV NS3 ELISA Kit (BioFront Technologies Inc.; Tallahassee, FL). Protein levels were compared to a standard curve, and both assays have lower limits of detection of 1 ng/mL.
Inhibition of HCV replication
To evaluate cellular factors involved in viral entry, anti-CD81 monoclonal antibody (MA1–80820; Thermo Scientific) or IgG isotype control antibody (A3812; Sigma-Aldrich) were incubated with islets or Huh7.5 cells 1 day prior to and during HCV infection at dilutions of 1:100 and 1:2500. Incubation with 0.1 ng or 1000 ng of consensus interferon (Infergen® from Three Rivers Pharmaceuticals, LLC; Cranberry Township, PA) was performed 1 day before and during viral infection. A single dose of sofosbuvir (Gilead) was added at a concentration of 0.25 mg/mL.
Cytokine expression
Cells were infected as described above, and culture supernatants were collected at days 1, 3, and 7 post-infection. IL-6, IL-8, TNFα, IL-1β, IL-12(p40), IL-17, and IFNα were measured using the Luminex multiplex assay (EMD Millipore Co; Bilerica, MA) with a lower limit of detection of 3.2 pmol/mL.
MicroRNA-122 (miR-122) expression
Total RNA was extracted from islets and Huh7.5 cells as described above. Reverse transcription was performed with 40 ng of total RNA using the TaqMan microRNA reverse transcription kit (Applied Biosystems; Foster City, CA), a specific RT primer for miR-122 (UGG AGU GUG ACA AUG GUG UUU G), and the endogenous control miR-16 (UAG CAG CAC GUA AAU AUU GGC G). Therefore, only the miRNAs of interest were reverse transcribed into cDNA. cDNA was amplified using miRNA-specific PCR primers provided in the TaqMan microRNA assay and the TaqMan Universal PCR master mix with uracil N-glycosylase. Results were quantified using the Applied Biosystems 7300 Real-Time PCR system and expressed using the ΔΔCt method.
Statistical analysis
Data are expressed as mean + SEM. Means were tested for statistical significance using the Student’s t-test. A significance level of p < 0.05 was applied when comparing virus-treated with untreated cells. Statistical analyses were performed using GraphPad Prism 5 (San Diego, CA).
Discussion
While significant advances have been made in the treatment of HCV in recent years, direct-acting agents are costly and not available to many individuals. Moreover, the focus on liver disease as the sole outcome of HCV replication is limited in scope and neglects the extrahepatic complications of this systemic infection. A large number of studies confirm an increased risk for T2D in patients with chronic HCV infection (reviewed in [
6]). It is estimated that ~30% of patients with liver cirrhosis develop diabetes [
25]. A meta-analysis of 34 studies reported a significantly higher risk of T2D in patients with HCV compared to patients with hepatitis B virus, matched controls, or patients with other forms of liver disease [
26].
While HCV is hepatotropic, evidence of extrahepatic replication has been identified in a variety of non-hepatic tissues, including the thyroid, bone marrow, adrenal gland, spleen, lymph node, cervicovaginal fluid, and brain [
10‐
12,
27‐
31]. Moreover, HCV RNA, viral antigens, and/or virus-like particles have been detected in the pancreata of patients with chronic HCV, highly suggestive of viral infection in vivo [
10,
11,
13]. Nonetheless, direct evidence of viral replication in islets is lacking because of the difficulty in procuring the appropriate clinical samples. Wang et al. previously reported low-level HCV replication within the insulin-producing beta cell line MIN6 [
32], although that cell line was murine in origin, infectious progeny virions were not released, and the authors utilized a higher multiplicity of infection than in the current study with human islets. Thus, we provide the first evidence of direct viral infection of pancreatic islet cells in vitro. Notably, viral infection was demonstrated in islets from multiple donors.
In this pilot study, infection in pancreatic islets was demonstrated using multiple complementary assays, including qualitative strand-specific RT-PCR for HCV RNA and quantitative ELISA for two HCV proteins. HCV replication was evaluated further by demonstrating inhibition with anti-CD81 antibodies anti-SR-B1 antibodies, sofosbuvir – a potent HCV polymerase inhibitor – and the antiviral cytokine IFNα. Viral RNA and protein were absent in uninfected islets, and a non-permissive cell line (293 T) showed no evidence of viral RNA or protein after in vitro exposure. We have previously shown detection of the NS5A non-structural protein by Western Blot in an extrahepatic cell type [
16]; however the number of islets available prevented a similar approach here. The production of infectious virions in islet cells that were capable of subsequent rounds of infection was also shown.
miR-122 is highly abundant in the human liver and represents a determinant of efficient HCV replication in hepatocytes. miR-122 also plays an important role in regulating lipid homeostasis [
33]. While not absolutely required, exogenous expression of miR-122 enhances HCV replication in non-hepatic cells [
34,
35]. Other have reported decreased miR-122 expression in pancreatic cancer compared to healthy tissues [
36] and a positive association between miR-122 mRNA levels in islets and insulin mRNA biosynthesis [
37]. Given the critical role of lipids in the HCV life cycle, miR-122 regulation may impact viral replication in extrahepatic cell types as well and deserves additional investigation. Moreover, direct comparison of HCV replication levels in islets and primary hepatocytes from the same donor have not been performed; thus, the relevant permissiveness of these two cells types to infection cannot be compared directly. Finally, JFH1 is a genotype 2a strain of HCV, and the ability of other HCV genotypes to replicate within islet cells has not been explored yet.
It is important to note that while all islet cell preparations from multiple donors could be infected with HCV, the level of infection achieved showed donor-dependent variability suggesting that cellular factors likely play a critical role in the amount of viral replication that occurs. This in vitro finding may also partially explain why only a subset of individuals with HCV infection develops diabetes and requires additional investigation. While infection appears to be CD81-dependent and SR-B1-dependent, the role of other HCV entry factors have not been evaluated in the pancreas to date and should be investigated carefully in future studies and may require evaluation of multiple donors since replication levels may be dependent on donor-specific levels of a particular entry factor or a combination of entry factors. While we did quantify the mRNA levels of several HCV receptors, protein levels remain to be evaluated in islets. Additionally, other than the NS5B inhibitor sofosbuvir, the role of other direct-acting agents on replication within the pancreas was not evaluated. Finally, the concentration of sofosbuvir used was relatively high, although it is unclear if a higher sofosbuvir dose would efficiently eliminate in virus replication in the pancreas.
The mechanisms by which HCV may promote T2D include impairment of the insulin signaling pathway by viral proteins and HCV-induced liver inflammation resulting in the release of pro-inflammatory cytokines and chemokines that interfere with insulin signaling [
38]. HCV can have direct effects on insulin signaling. Additionally, TNFα may play a pivotal role in the association between HCV infection and diabetes (reviewed in [
6]). Nonetheless, the mechanisms by which HCV contributes to T2D may be distinct from previously characterized mechanisms of type 1 diabetes or T2D, and previous studies did not evaluate functional interactions between HCV and islet cells. Interestingly, we found that HCV infection of islet cells triggered altered cell expression of pro-inflammatory cytokines suggesting a possible connection between viral replication and islet cell function, although this deserves additional investigation. This represents an important model for studies of HCV-islet interactions by closely mimicking the in vitro physiologic environment.
Conclusions
These findings are preliminary in nature. Nonetheless, they imply that direct infection of beta cells by HCV may play a key role in the association between HCV and type 2 diabetes. However, it should be noted that the exact cell that is infected by HCV is not critical to our hypothesis. We believe that the virus infects beta cells based on their high expression levels of CD81, although other cellular factors should be evaluated rigorously in future studies. Regardless, even if the virus infects alpha and beta cells (or only alpha cells), the resulting inflammation and cytokine response may damage both alpha and beta cells resulting in islet dysfunction and diabetes. Nonetheless, this is a pilot study, and these preliminary results should be viewed with caution until confirmed by others using complementary methodologies. Understanding the etiology of diabetes in individuals with HCV infection may enable the development of novel treatment modalities and prevention strategies based on the specific viral mechanisms that trigger diabetes. These results support the hypothesis that the pancreas serves as an extrahepatic reservoir of replication and that HCV treatment with direct-acting agents may ameliorate virus-mediated diabetes.